* Modules/fetch/FetchHeaders.idl:
Replace [JSBuiltinConstructor] with real constructor. Keep other builtin
attributes as they are still used by other Fetch code.

* bindings/js/JSDOMConvertRecord.h:
Fix record conversion to work with proxies by changing to use the method table
for getOwnPropertyNames, and undefined values by not excluding undefined values.

LayoutTests:

* fetch/header-constructor-is-array-expected.txt:
* fetch/header-constructor-is-array.html:
Update test to match spec. An array with out a prototype will not yield
a valid Header as it is not iterable.